Saturn will reach opposition, when it lies opposite to the Sun in the sky. Lying in the constellation Capricornus, it will be visible for much of the night, reaching its highest point in the sky around midnight local time.
From Cambridge, it will be visible between 21:07 and 04:33. It will become accessible at around 21:07, when it rises to an altitude of 10° above your south-eastern horizon. It will reach its highest point in the sky at 00:50, 29° above your southern horizon. It will become inaccessible at around 04:33 when it sinks below 10° above your south-western horizon.
2021 apparition of Saturn
|23 May 2021||–||Saturn enters retrograde motion|
|02 Aug 2021||–||Saturn at opposition|
|10 Oct 2021||–||Saturn ends retrograde motion|
A close approach to the Earth
At around the same time that Saturn passes opposition, it also makes its closest approach to the Earth – termed its perigee – making it appear at its brightest and largest.
This happens because when Saturn lies opposite to the Sun in the sky, the Earth passes between Saturn and the Sun. The solar system is lined up with Saturn and the Earth on the same side of the Sun, as shown by the configuration labelled perigee in the diagram below:
The panels below show a comparison of the apparent size of Saturn when seen at opposition in 2021, and when it is most distant from the Earth at solar conjunction.
A comparison of the size of Saturn as seen at 2021 opposition and at solar conjunction.
In practice, however, Saturn orbits much further out in the solar system than the Earth – at an average distance from the Sun of 9.54 times that of the Earth, and so its angular size does not vary much as it cycles between opposition and solar conjunction.
The rings of Saturn
Saturn will be angled to show its northern hemisphere at this opposition, and the rings will inclined at an angle of 18° to our line of sight, making them very well presented.
The graph below shows the changing inclination of Saturn's rings over time. The black line indicates their inclination to our line of sight from the Earth. A negative angle indicates that the north pole is tipped towards us, while a positive angle indicates that we see the south pole. A angle close to zero means that Saturn's rings appear close to edge on.
The red line indicates the inclination of the rings to the Sun's line of sight to the planet. Interesting phenomena can occur when the rings are very close to edge-on, if the Sun illuminates one side of the rings, while we see the other. At such times, we see the unilluminated side of the rings.
The Seeliger Effect
For a few hours around the exact moment of opposition, it may be possible to discern a marked brightening of Saturn's rings in comparison to the planet's disk, known as the Seeliger Effect.
This occurs because Saturn's rings are made of a fine sea of ice particles which are normally illuminated by the Sun at a slightly different angle from our viewing angle, so that we see some illuminated particles and some which are in the shadow of others.
At around the time of opposition, however, the ice particles are illuminated from almost exactly the same direction from which we view them, meaning that we see very few which are in shadow.
At opposition, Saturn is visible for much of the night. When it lies opposite to the Sun in the sky, this means that it rises at around the time the Sun sets, and it sets at around the time the Sun rises. It reaches its highest point in the sky at around midnight local time.
But even when it is at its closest point to the Earth, it is not possible to distinguish it as more than a star-like point of light without the aid of a telescope.
At the moment of opposition, Saturn will lie at a distance of 8.94 AU, and its disk will measure 18.6 arcsec in diameter, shining at magnitude 0.2. Its celestial coordinates at the moment it passes opposition will be:
|Object||Right Ascension||Declination||Constellation||Magnitude||Angular Size|
The coordinates above are given in J2000.0.
Over the weeks following its opposition, Saturn will reach its highest point in the sky four minutes earlier each night, gradually receding from the pre-dawn morning sky while remaining visible in the evening sky for a few months.
The sky on 2 Aug 2021
|The sky on 2 August 2021|
23 days old
All times shown in EDT.
The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).
This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.
|02 Aug 2021||– Saturn at opposition|
|14 Aug 2022||– Saturn at opposition|
|27 Aug 2023||– Saturn at opposition|
|08 Sep 2024||– Saturn at opposition|